Photopolymerizable organopolysiloxane compositions having SiC-bonded acryloxyalkyl or methacryloxyalkyl groups and mercaptoalkyl groups are described in U.S. Pat. No. 4,290,869 to Pigeon. In these compositions, silanes are present in addition to organopolysiloxanes, because in preparing the constituents used in the preparation of the compositions, at most only 45 percent by weight of the theoretical amount of alkanol is released. Pigeon also discloses at column 8, lines 36 to 38, that the compositions described in the patent have a shelf life of about 48 hours in closed, opaque containers.
In contrast to the compositions described in U.S. Pat. No. 4,290,869, the compositions of this invention are free of, or at least have fewer Si-bonded hydroxyl groups in the R.sub.2 SiO units and are free of, or at least have fewer organic titanium or tin compounds than the compositions described by Pigeon. Moreover, the compositions of the present invention contain units of the formula R.sub.3 SiO.sub.1/2, whereas these units are absent from the compositions of Pigeon.
Therefore, it is an object of the present invention to prepare compositions from readily available materials which can be stored in sealed containers with the exclusion of light, especially ultraviolet light, for at least 6 months. Another object of the present invention is to prepare compositions which are of sufficiently low viscosity prior to their crosslinking to be capable of penetrating even into comparatively narrow gaps. Another object of the present invention is to provide one-component systems which do not crosslink in the apparatus used for their final processing when the apparatus is shut-down for a period of time and thus obviates cleaning of the apparatus. Still another object of the present invention is to provide compositions which do not give off an unpleasant odor and crosslink rapidly under the influence of ultraviolet light at room temperature or at temperatures not exceeding 100.degree. C. Still another object of the present invention is to provide compositions in which crosslinking is not impaired by atmospheric oxygen to an extent which appreciably affects encapsulating of electronic components; and which, on crosslinking, yield products that are transparent and have a soft and gel-like consistency. A further object of the present invention is to provide compositions that are "self-sealing", that is, when they have been cut through, for example, and the parts separated from one another by the cut are sufficiently close to one another, that they flow together again. A still further object of the present invention is to provide compositions in which the crosslinked products do not flow off the substrates on which these products have been formed. A still further object of the present invention is to provide compositions which are extremely well suited for encapsulating electronic components. A still further object of the present invention is to provide compositions that are substantially free of Si-bonded hydroxyl groups, titanium compounds and tin compounds, but crosslink rapidly when exposed to ultraviolet light.